Baseband I and Q converter and method for performing baseband I and Q conversion

ABSTRACT

Intermediate or radio frequency signals are band pass filtered by two band pass filters having identical frequencies, but 90 degree relative phase shifts at the output, with a reduced sampling rate. The outputs of the two BPFs represent in-phase (I) and quadrature-phase (Q) baseband signals needed for demodulation.

REFERENCE TO PROVISIONAL APPLICATIONS

[0001] This application claims the benefit of the U.S. ProvisionalApplication No. 60/224,950, filed Aug. 11, 2000.

TECHNICAL FIELD

[0002] The present invention relates to methods and apparatus forconversion of radio signals, and more particularly, to method andapparatus for conversion of intermediate frequencies to basebandfrequencies in a radio receiver.

BACKGROUND OF THE INVENTION

[0003] In the prior art it is known to convert an intermediate frequency(IF) signal to baseband signals by down-sampling the output of a bandpass filter (BPF) having a delayed output and a non-delayed output. Thisis shown in FIG. 1, which is a block diagram of a signal converter knownin the prior art.

[0004] The converter 100 receives an analog signal at the input to ananalog-to-digital (A/D) converter 102 which operates at a samplingfrequency F_(s0) The digital signals from the A/D converter 102 passthrough a band pass frequency (BPF) 104, whose output is passed througha 1/N down sampler 106 on input 108. The output of the BPF 104 is alsopassed through a delay element 110, whose output is passed through the1/N down sampler 106 on input 112. The delay created by the delayelement 110 must be exactly ¼ of the sampling time period of the carrierfrequency of the signal (i.e., 1/(4f₀), where f₀ is the carrierfrequency of the signal) to reflect the 90 degree phase shift.

[0005] The 1/N down sampler 106 has two samplers—one for each of theinputs 108 and 112. The outputs of the samplers are respectively placedon outputs 114 and 116 of the 1/N down sampler 106. The outputs are thenpassed through the two respective low pass filters (LPFs) 118 and 120 togenerate the I and Q signals.

[0006] It is inherently difficult to generate an exact clock delay withthe desired resolution in a digital system. Accordingly, it is desirableto implement a converter in a manner that does not require theproduction of a precise delay.

SUMMARY OF THE INVENTION

[0007] According to one aspect, the invention is a converter forconverting an intermediate frequency (IF) signal to a baseband signal,the IF signal having a center frequency of f₀ and bandwidth R. Theconverter includes a Σ-Δ A/D converter for converting the IF signal toan output signal by sampling the IF signal at a sampling rate F_(s0),where F_(s0)=f₀/N (N an integer) and F_(s0)>R. The converter alsoincludes a first band pass filter for producing an I signal, the firstband pass filter including a first finite-impulse response (FIR) filteroperating at a sampling rate F_(s2), where R≦F_(s2)<F_(s0); and a secondband pass filter for producing a Q signal, the second band pass filterincluding a second FIR filter operating at the sampling rate F_(s2),such that the phases of the I and Q signals differ by 90 degrees.

[0008] According to another aspect, the invention is a method forconverting an intermediate frequency (IF) signal to a baseband signal,the IF signal having a center frequency of f₀ and bandwidth R. Themethod includes the step of a) converting the IF signal to an outputsignal by using a Σ-Δ A/D converter for sampling the IF signal at asampling rate F_(s0), where F_(s0)=f_(s0)/N (N an integer) and F_(s0)>R.The method also includes the step of b) producing an I signal by passingthe output signal through a first band pass filter including a firstfinite-impulse response (FIR) filter operating at a sampling rateF_(s2), where R≦F_(s2)<F_(s0). The method also includes the step of c)producing a Q signal by passing the output signal through a second bandpass filter including a second FIR filter operating at the sampling rateF_(s2), such that the phases of the I and Q signals differ by 90degrees.

[0009] According to yet another aspect, the invention is a converter forconverting an intermediate frequency (IF) signal to a baseband signal,the IF signal having a center frequency of f₀ and bandwidth R. Theconverter includes means for converting the IF signal to an outputsignal by using a Σ-Δ A/D converter for sampling the IF signal at asampling rate F_(s0), where F_(s0)−f₀/N (N an integer) and F_(s0)>R. Theconverter also includes means for producing an I signal by passing theoutput signal through a first band pass filter including a firstfinite-impulse response (FIR) filter operating at a sampling rateF_(s2), where R≦F_(s2)<F_(s0). The converter further includes means forproducing a Q signal by passing the output signal through a second bandpass filter including a second FIR filter operating at the sampling rateF_(s2), such that the phases of the I and Q signals differ by 90degrees.

BRIEF DESCRIPTION OF TRE DRAWINGS

[0010]FIG. 1 is a block diagram of an I and Q converter known in theprior art.

[0011]FIG. 2 is a block diagram of a first preferred embodiment of animproved I and Q converter in accordance with the present invention.

[0012]FIG. 3 is a block diagram of a second preferred embodiment of animproved I and Q converter in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

[0013]FIG. 2 is a block diagram of a first preferred embodiment of animproved I and Q converter in accordance with the present invention.This converter 250 can be used to convert from an IF to baseband I and Qsignals. The converter 250 includes a sigma-delta analog-to-digital (Σ-ΔA/D) converter 202, and BPF filter blocks 220 and 240. The BPF filterblocks 220 and 240 receive their inputs from the Σ-Δ A/D converter 202,which operates at a sampling rate F_(s0).

[0014] In operation, the converter 250 receives an incoming signal 200having a center frequency of f₀ and bandwidth R. The incoming signal 200is digitized by Σ-Δ A/D converter 202 at a sampling rate F_(s0),F_(s0)=f₀/N and F_(s0)>R. The output of Σ-Δ A/D converter 202 is bandpass filtered by the BPF filter blocks 220 and 240. Each of the BPFfilter blocks 220 and 240 includes a finite-impulse response (FIR)filter. BPF filter block 220 includes the FIR filter 222 (FIR-i), andBPF filter block 240 includes the FIR filter 242 (FIR-q). The BPF filterblocks 220 and 240 operate at the sampling rate F_(s2), R≦F_(s2)<F_(s0),and respectively generate the I and Q signals 226 and 246.

[0015] The BPF filter blocks 220 and 240 have identical frequencyresponses, except that their time domain responses are 90 degrees out ofphase. The FIR filter 222 can be designed by using a typical low passfilter (LPF) design process to obtain the impulse response of the FIR.The bandwidth of the LPF is greater than or equal to R/2. Next, the LPFimpulse response is multiplied by a sine function having a centerfrequency of f₀.

[0016] As an example of typical LPF design, a brick wall filter can bedesigned using a SINC function (sin x/x), and the resulting impulseresponse is truncated by a conventional window function such as theHamming window.

[0017] The FIR filter 242 can designed with the same procedure exceptthat the LPF impulses response is multiplied by a cosine function havinga center frequency of f₀.

[0018] Depending on the bandwidth of FIR filters 222 and 224, additionalbandlimiting can be provided by post demodulation LPFs.

[0019]FIG. 3 is a block diagram of a second preferred embodiment of animproved I and Q converter in accordance with the present invention. Theconverter 350 is very similar to the converter 250 shown in FIG. 2. Theconverter 350 includes a sigma-delta analog-to-digital (Σ-Δ A/D)converter 302, and BPF filter blocks 320 and 340. The BPF filter blocks320 and 340 receive their inputs from the Σ-Δ A/D converter 302, whichoperates at a sampling rate F_(s0).

[0020] In operation, the converter 350 receives an incoming signal 200having a center frequency of f₀ and bandwidth R. The incoming signal 200is digitized by Σ-Δ A/D converter 302. The output of Σ-Δ A/D converter302 is band pass filtered by the BPF filter blocks 320 and 340. Each ofthe BPF filter blocks 320 and 340 includes a finite-impulse response(FIR) filter and an infinite-impulse response (IIR) filter. BPF filterblock 320 includes the FIR filter 322 (FIR-i) and the IIR filter 324.The BPF filter block 340 includes the FIR filter 342 (FIR-q) and the IIRfilter 344.

[0021] The IIR filters 324 and 344 reduce the computational powerrequired to implement the BPF blocks 320 and 340. The signal 304entering the BPF block 320 is first filtered by the FIR BPF 322 for aninitial bandwidth reduction. The bandwidth reduced signal is downsampled at F_(s1) samples per second and filtered by the IIR BPF 324.Further bandwidth reduction by the IIR BPF 324 allows the signal to bedown sampled to produce output signal 324 at the final sampling rateF_(s2). The BPF block 340 is implemented in a similar manner except thatFIRs 322 and 342 are designed with the earlier mentioned principle toproduce output with 90 degrees phase shifts.

[0022] The BPF filter blocks 320 and 340 operate at the sampling rateF_(s2) and respectively generate the I and Q signals 326 and 346.

[0023] Although the present invention is described as using a Σ-Δ A/Dconverter, it works well with all types of A/D converters. Advantages ofusing a Σ-Δ A/D converter include its oversampling and one-bit output.

[0024] While the foregoing is a detailed description of the preferredembodiment of the invention, there are many alternative embodiments ofthe invention that would occur to those skilled in the art and which arewithin the scope of the present invention.

1. A converter for converting an intermediate frequency (IF) signal to abaseband signal, the IF signal having a center frequency of f₀ andbandwidth R, comprising: a Σ-Δ A/D converter for converting the IFsignal to an output signal by sampling the IF signal at a sampling rateF_(s0), where F_(s0)=f₀/N (N an integer) and F_(s0)>R; a first band passfilter for producing an I signal, the first band pass filter including afirst finite-impulse response (FIR) filter operating at a sampling rateF_(s2), where R≦F_(s2)<F_(s0); and a second band pass filter forproducing a Q signal, the second band pass filter including a second FIRfilter operating at the sampling rate F_(s2), such that the phases ofthe I and Q signals differ by 90 degrees.
 2. The converter of claim 1,wherein the first and second band pass filters are designed as low passfilters having bandwidths greater than or equal to R/2 and havingimpulse responses, respectively, the impulse response of the low passfilter design of the first band pass filter being multiplied by a sinefunction having a center frequency of f₀, and the impulse response ofthe low pass filter design of the second band pass filter beingmultiplied by a cosine function having a center frequency of f₀.
 3. Theconverter of claim 2, wherein the first and second band pass filters aredesigned as low pass filters using a SINC function.
 4. The converter ofclaim 2, wherein the impulse responses of the first and second band passfilters are truncated by a conventional window function.
 5. Theconverter of claim 4, wherein the window function is the Hamming windowfunction.
 6. A method for converting an intermediate frequency (IF)signal to a baseband signal, the IF signal having a center frequency off₀ and bandwidth R, comprising the steps of: a) converting the IF signalto an output signal by using a Σ-Δ A/D converter for sampling the IFsignal at a sampling rate F_(s0), where F_(s0)=f₀/N (N an integer) andF_(s0)>R; b) producing an I signal by passing the output signal througha first band pass filter including a first finite-impulse response (FIR)filter operating at a sampling rate F_(s2), where R≦F_(s2)<F_(s0); andC) producing a Q signal by passing the output signal through a secondband pass filter including a second FIR filter operating at the samplingrate F_(s2), such that the phases of the I and Q signals differ by 90degrees.
 7. The method of claim 6, wherein step b) includes designingthe first band pass filter as a low pass filter having a bandwidthgreater than or equal to R/2 and having a first impulse response that ismultiplied by a sine function having a center frequency of f₀, and stepc) includes designing the second band pass filter as a low pass filterhaving a bandwidth greater than or equal to R/2 and having a secondimpulse response that is multiplied by a cosine function having a centerfrequency of f₀.
 8. The method of claim 7, wherein steps b) and c)further include respectively designing the first and second band passfilters as low pass filters using a SINC function.
 9. The method ofclaim 7, wherein steps b) and c) further include respectively truncatingthe impulse responses of the first and second band pass filters by aconventional window function.
 10. The method of claim 9, wherein stepsb) and c) include respectively truncating the impulse responses of thefirst and second band pass filters by Hamming window functions.
 11. Aconverter for converting an intermediate frequency (IF) signal to abaseband signal, the IF signal having a center frequency of f₀ andbandwidth R, comprising: means for converting the IF signal to an outputsignal by using a Σ-Δ A/D converter for sampling the IF signal at asampling rate F_(s0), where F_(s0)=f₀/N (N an integer) and F_(s0)>R;means for producing an I signal by passing the output signal through afirst band pass filter including a first finite-impulse response (FIR)filter operating at a sampling rate F_(s2), where R≦F_(s2)<F_(s0); andmeans for producing a Q signal by passing the output signal through asecond band pass filter including a second FIR filter operating at thesampling rate F_(s2), such that the phases of the I and Q signals differby 90 degrees.
 12. The converter of claim 11, wherein the means forproducing an I signal includes a first low pass filter having abandwidth greater than or equal to R/2 and having a first impulseresponse that is multiplied by a sine function having a center frequencyof f₀, and the means for producing a Q signal includes a second low passfilter having a bandwidth greater than or equal to R/2 and having asecond impulse response that is multiplied by a cosine function having acenter frequency of f₀.
 13. The converter of claim 12, wherein the meansfor producing an I signal and for producing a Q signal further includelow pass filters using a SINC function.
 14. The converter of claim 12,wherein the means for producing an I signal and for producing a Q signalfurther include respectively means for truncating the impulse responsesof the first and second band pass filters by a conventional windowfunction.
 15. The converter of claim 14, wherein the means for producingan I signal and for producing a Q signal respectively includes means fortruncating the impulse responses of the first and second band passfilters by Hamming window functions.